MSVC2008 Compile fix

 Fix for compiler errors in Visual Studio 2008.

## Bug##
Visual Studio 2008 errors when compiling about Extension.size() in Gwen::Min() returning an ambiguous value (int vs. unsigned).

## Fix##
Replaced the hardcoded values 512 on the offending lines with sizeof(FilterBuffer) since Extension.size()and sizeof() both return a size_t.

## Misc Changes##
Changed some hard-coded values used multiple times into a defined value. This should makes the code more readable and less prone to errors if the values need to change.